Method of treating an electrode,of which at least one portion contains thorium oxide in its surface



May 5, 1970 A. TAXIL 3,510,190 METHOD OF TREATING AN ELECTRODE, OF WHICH AT LEAST ONE PORTION CONTAINS THORIUM OXIDE IN ITS SURFACE Filed Jan. 30, 1967 2 Sheets-Sheet 1 Ava/m? May 5, 1970 A TAxlL 3,510,190

METHOD OF TREATING AN ELECTRODE, OF WHICH AT LEAST ONE PORTION cONTAINs THORIUM OXIDE IN ITS SURFACE Filed Jan. 30, 1967 2 Sheets-Sheet 2 34 4 Fig.3

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United States Patent 3,510,190 METHOD OF TREATING AN ELECTRODE, OF WHICH AT LEAST ONE PORTION CONTAINS THORIUM OXIDE IN ITS SURFACE Andr Taxil, Reuil-Malmaison, France, assignor to Claude Paz et Visseaux Filed Jan. 30, 1967, Ser. No. 612,506 Claims priority, applicgigrg rance, Apr. 18, 1966,

Int. or. from 9/04 US. Cl. 316-26 9 Claims ABSTRACT OF THE DISCLOSURE The invention re ates to a method of treating an electrode, of which at least one portion contains thorium oxide in its surface, by heating in an atmosphere containing a hydrocarbon.

It is known to carburize thoriated-tungsten cathodes of vacuum tubes to provide a carbon impregnated tungsten cathode; this operation increases the thermionic emission and the carbon which it deposits serves as a gettering agent during the operation of the tube. The methods employed for this carburizing treatment are somewhat dlfficult to carry out and constitute an additional operation in the manufacture proper of the electrode.

One object of the invention is to provide a method for carburizing thoriated electrodes, which method is simple to carry out and also effects the sintering of the emissive deposit when the manufacture of the electrode comprises such sintering.

Another object of the invention is to provide a method for carburizing thoriated electrodes intended to operate in an ionisable atmosphere.

A feature of the invention is to provide a method for carburizing thoriated electrodes, which method comprises the passage of an electric arc discharge between two electrodes, one of which is the electrode to be treated, the atmosphere for this discharge containing hydrocarbon at a partial pressure of the order of a millimetre of mercury.

Embodiments of the invention are diagrammatically illustrated by way of nonlimiting examples in the accompanying figures in which:

FIG. 1 shows a device for carrying out the method of the invention.

FIG. 2 shows a discharge lamp provided with two electrodes according to the invention.

FIGS. 3, 4, 5 and 6, respectively, show four other-electrodes which have undergone the method of treatment according to the invention.

The electrode treated in the device of FIG. 1 is intended for a 400-watt lamp operating by discharge in an atmosphere under pressure containing mercury and halides; it conforms to French Pat. No. 1,464,066. This electrode is composed essentially of:

A rod 4 of thoriated tungsten containing 1.6% of thorium oxide, of a length of 15 mm. and a diameter of 0.75 mm.;

A helix 6 comprising approximately seven contiguous or almost contiguous turns, consisting of a tungsten filament 0.5 mm. in diameter;

Patented May 5, 1970 ice A coating 8 on the helix and on the neighbouring zones of the rod 4, more particularly its end, this coating being composed of equal weights of thoria powder and of thorium powder and a little temporary binder.

The electrode is held by its stem in a resilient collet 10 and is enclosed with a bell 12 having a branch 14, in which bell there is sealed a tungsten rod 16. The bell is, for example, made of borosilicate glass, called Pyrex.

For the treatment by which the invention is characterized, there is passed into the bell by way of its branch commercially pure argon, which has been brought into contact with naphthalene grains at ambient temperature; the partial pressure of this hydrocarbon is then of the order of a thousandth of an atmosphere.

When the bell has been filled with this mixture, the electrode -4, 6, 8 and the rod 16 are connected respectively to the poles of a direct-current or alternating-current source of appropriate voltage and in series with a resistance.

By bringing the two electrodes into contact and then moving them apart, an arc discharge is produced. For an electrode having the above-indicated dimensions, the current source and the resistance in series therewith are such that the discharge has a current of 8 amperes. For an elec trode comprising a rod of a diameter of 1.2 mm. and a helix consisting of filament having a diameter of 0.75 mm. a discharge of about 12 amperes would be passed; this electrode is intended, for example, for a Z-kilowatt lamp supplied by a 380-volt alternating-current source.

This discharge is passed for about twenty seconds while the e ectrode 4, 6, 8 is moved, so that the cathode spot is successively produced on all the points of the deposit 8. The spot brings the point at which it is formed to about 2000 C., whereby the thoria and the thorium are converted into a ceramic and the binder is eliminated at this point; since the cathode spot is highly localised and this stage of the treatment is short, the mass of the electrode does not by far reach this temperature.

After this discharge, the electrode is allowed to cool for about one minute in argon without naphthalene. Thereafter, the arc discharge is renewed and accompanied by displacements of the electrode to be treated, but in argon without naphthalene, except in involuntary traces. At the end of about twenty seconds, the electric discharge is extinguished and, after argon without naphthalene has been passed through for one minute, whereby the electrode is cooled, the bell is removed and the electrode is withdrawn from its collet 10.

It is generally desirable to precede the passage of the argon containing naphthalene by a purge of the hell by means of pure argon.

Two electrodes intended for a 2-kilowatt 380-volt lamp treated as described with reference to the drawings were fitted in a manner known for high-pressure mercury vapour lamps, in a silica tube of a diameter of 30 mm.; the distance between these electrodes, called main electrodes, was mm. There were also fitted in the tube two auxiliary electrodes, each close to one of the main electrodes and connected to the other main electrode by a resistance of 20,000 ohms.

To form the discharge atmosphere, there were introduced into the tube:

argon under a pressure (at ambient temperature) of 20 mm. Hg;

0.26 g. of mercury;

0.150 g. of a mixture of the iodides of sodium, thallium and indium.

A number of tubes produced in this way were tested. Their alternating-current starting voltage was found to be of the order of 300 to 350 v. depending upon the tubes; in the case of the reference tubes, which were identical ex- 3 cept that their electrodes had not been subjected to the treatment according to the invention the starting voltage was always above 380 v.

FIG. 2 illustrates a lamp of this type with two main electrodes 20, 22 but a single auxiliary electrode 24. The discharge tube 26 is surrounded in known manner by an envelope 28, which is exhausted. There is shown at 30 the mercury and at 32 a spot formed by the halides before the lamp has been put into operation.

The method of the invention may be carried out in manners different from that described in the foregoing.

For example, the naphthalene may 'be replaced by a more volatile hydrocarbon, the vapour pressure of which may, if desired be lowered by cooling. Conversely, the naphthalene may be replaced by a less volatile hydrocarbon, which is heated if the vapour pressure is found to be insuflicient. The discharge in an atmosphere containing this hydrocarbon may also be maintained for a longer time. It is also possible to employ a number of hydrocarbons simultaneously.

The treated electrode may be different both in the composition of the mixture containing the thorium oxide and in the metallic support for this mixture. For example, there may be employed a helix 6 having noncontiguous turns for increasing the quantity of emissive products. The mixture may comprise other proportions of thoria and thorium. It may contain other constituents, for example silica.

For example, all or part of the portion whose surface contains thorium oxide may be surrounded, after the treatment, with a metallic member not comprising any emissive material, but formed with voids, slots or pores, for example. Electrons will pass through these voids, as also a little metallic thorium, which will activate the said member. The said member is, for example, a second tungsten helix of the same pitch and the same filament diameter as the helix 6.

This member protects the emissive deposit from bombardment, reduces the temperature of the electrode during operation and in addition increases the thermal inertia of the electrode and its cooling time, so that no halides are deposited on the electrode during the extinction.

In FIGS. 3 and 4, the said member consists of a helix 34 or 36 of tungsten filament, screwed onto the helix 6 and extending beyond it in the forward direction (FIG. 3) or t in the rearward direction (FIG. 4). The said member could also extend beyond the helix 6 both in the forward direction and in the rearward direction or not extend beyond it at all.

FIGS. 5 and 6 are variants of "FIGS. 3 and 4, in which the helix 6 is replaced by two shorter helices 38, 40, de-

4 fining a small void in which an additional quantity of emissive material is lodged.

Further modifications may be made without departing from the scope of the present invention.

What I claim is:

1. A method of treating a thoriated tungsten electrode, of which at least one portion contains thorium oxide in its surface, comprising striking an electric arc between two spaced electrodes of which one is the electrode to be treated, and maintaining an atmosphere for the arc containing an inert gas and a small amount of hydrocarbon vapor to carburize said electrode.

2. A method as claimed in claim 1, the partial pressure of said hydrocarbon vapor being about one millimeter of mercury.

3. A method as claimed in claim 1, said inert gas being at a pressure of about one atmosphere.

4. A method as claimed in claim 1, said inert gas being argon.

5. A method as claimed in claim 1, and thereafter cooling said treated electrode in an inert atmosphere substantially free from hydrocarbon vapor.

6. A method as claimed in claim 1, and thereafter cool ing said treated electrode, striking a further electric arc for which said treated electrode serves as a cathode, and maintaining an atmosphere for said further arc of an inert gas substantially free from hydrocarbon vapor.

7. A method as claimed in claim 1, including varying the position of the two electrodes relative to each other to produce said electric arc at different portions on said treated electrode.

8. A method as claimed in claim 1, said surface consisting essentially of a mixture of thoria and metallic thorium.

9. A method as claimed in claim 1, said hydrocarbon being naphthalene.

References Cited UNITED STATES PATENTS 1,704,981 3/1929 Lebrun 31626 2,401,040 5/1946 Becker 31626 2,591,474 4/ 1952 Stutsman 316--26 2,324,559 7/1943 Cooke 3l 626 JOHN F. CAMPBELL, Primary Examiner R. I. CRAIG, Assistant Examiner U.S. Cl. X.R. 2925 .17 

